The present invention relates generally to the field of spinal fixation devices. More specifically, the present invention is directed to a method and a system for spinal fixation which incorporate a polydirectional fixation screw with a biasing member and vertically offset linkages. The present invention is also directed to a method and system for spinal fixation which incorporates rods and polydirectional couplers.
The spinal column is a highly complex structure which houses and protects critical elements of the nervous system. In spite of these complexities, the spinal column is a highly flexible structure, capable of a high degree of curvature and twist through a wide range motion. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or threaten the critical elements of the nervous system housed within the spinal column.
A variety of systems has been disclosed in the art which achieve immobilization of portions of the spinal column by implanting artificial assemblies in or on the spinal column. These assemblies may be generally classified as anterior, posterior or lateral implants. Posterior implants are attached to the back of the spinal column generally by coupling to the pedicles with screws, or through hooks which attach under the lamina. In either case, the implants generally include elongate support rod elements which are coupled to the screws or hooks to immobilize several sequential vertebrae, for example to hold them stable so that adjacent bones may be fused with bone graft.
Various types of links or plates are also known for spinal fixation.
While known spinal fixation systems have proven to be useful for particular applications, they are all associated with drawbacks. In this regard, the fixation screws or hooks of most known systems are not able to adequately adjust in the sagittal plane or coronal plane. Overcoming this limitation typically requires time consuming reshaping of the rods or plates and links used to connect the hooks or screws. Such bending is a tedious process, which is inconsistent and adds unwanted time to an operation.
The drawbacks discussed above of prior spinal fixation systems are addressed to a significant degree by the SpineLink(trademark) system sold by the assignee of this application. The SpineLink(trademark) system includes a plurality of polydirectional screws which are connected by linkages. Sagittal plane adjustment is achieved through rotation of the polydirectional screw head, thereby substantially eliminating the need for significant contouring of the linkages.
The subject invention further improves upon the SpineLink(trademark) system.
According to one aspect, the present invention relates to an improved system for spinal fixation which includes a polydirectional fixation screw having a biasing member for normally biasing two partially spherical portions of a head of the screw apart.
According to another aspect, the present invention provides a connection member for adjustably interconnecting a polydirectional screw with a rod.
According to another aspect, the present invention relates to an improved system for spinal fixation which includes a plurality of vertically offset linkages.
According to a more particular aspect, the present invention relates to a connection member for adjustably interconnecting a polydirectional screw with a rod that has an arm radially offset from a longitudinal axis of the screw.
According to another more particular aspect, the present invention relates to a connection member for adjustably interconnecting a polydirectional screw with a rod that incorporates a ball engaging the rod which has a pair of lips or flanges to aid in compression and distraction.
An advantage of the present invention is to provide a system for spinal fixation that allows for adjustment in both the sagittal and coronal planes without the need to bend the connecting linkages.
Another advantage of the present invention is to provide a spinal fixation system including a polydirectional fixation screw having a biasing mechanism for maintaining a desired orientation between cooperating hemispherical portions of the screw during implantation.
In one form, the present invention provides a polydirectional fixation screw for use in connecting a bone with a linkage. The polydirectional fixation screw includes an upper member and a lower member. The upper member is attached to the linkage. The upper member includes a first hemispherical portion. The lower member engages the bone and includes a second hemispherical portion which cooperates with the first hemispherical portion to define a sphere. The second hemispherical portion is coupled to the first hemispherical portion for relative pivotal movement about an axis. The polydirectional fixation screw further includes a biasing member for biasing said first member away from said second member in a direction substantially parallel to the axis.
In another form, the present invention provides a system for spinal fixation. The system includes a rod, a polydirectional fixation screw, a connection member and a nut. The polydirectional fixation screw includes an upper member and a lower member. The upper member has a first hemispherical portion. The lower member includes a second hemispherical portion that cooperates with the first hemispherical portion to substantially define a sphere. The second hemispherical portion is coupled to the first hemispherical portion for relative pivotal movement about an axis. The connection member adjustably interconnects the polydirectional fixation screw and the rod. The connection member includes first and second flanges connected by an intermediate portion. The intermediate portion defines an aperture for receiving the rod. The second flange defines a recess for matingly receiving a portion of the sphere. The nut threadably engages the upper member and is rotatable in a first direction to arrest relative movement between the first and second hemispherical portions and also to arrest relative movement between the rod and the connection member.